The invention relates to thermal-radiation imaging devices having a detector element comprising a strip of semiconductor material in which free charge carriers can be generated by absorption of thermal radiation and in which an ambipolar drift of radiation-generated minority charge carriers can occur along the strip. The invention further relates to thermal-radiation imaging systems comprising such a device.
British Pat. No. 1,488,258 discloses a thermal-radiation imaging device comprising a strip of semiconductor material in which free charge carriers can be generated on absorption of thermal radiation incident on said strip. Biasing-electrode means are spaced in a direction along the strip for causing a bias current predominantly of majority charge carriers to flow along the strip. The bias current is capable of supporting an ambipolar drift of radiation-generated free minority charge carriers in the opposite direction to the bias current. Readout means are present in the ambipolar drift path between the spaced biasing-electrode means.
The semiconductor material of the strip is usually cadmium mercury telluride. The readout means may comprise, in close proximity, first and second readout electrodes forming ohmic contacts to the strip. These electrodes which may be of a metal such as aluminium extend across the strip in the known device, and one of these two electrodes may be common with a biasing electrode. The voltage developed between the two readout electrodes is a measure of the density of minority carriers generated by the radiation.
However, in another form, the readout means may be either metal or a semiconductor region (preferably extending across the strip) which forms a diode junction with the strip. This junction is reversebiased by applying a suitable bias voltage. The current generated via this diode is also a measure of the density of minority carriers generated by the radiation. The diode junctions may also be used in a nonbiased condition.
In the particular forms of this device which are described and shown in British Pat. No. 1,488,258, the metal or semiconductor region providing the readout means is mounted on (and confined to) the semiconductor strip. The strip itself is mounted in a conventional encapsulation arrangement for cooling the strip to the desired operating temperature and for providing the appropriate electrical connections. It is conventional practice to use wire bonds to provide the electrical connections to a thermal-radiation imaging device in such an encapsulation. However the bonding of a wire connection directly to such a readout means on the device strip can cause problems. The area of the readout means is a sensitive area in the ambipolar drift path. Wire-bonding in this area can introduce damage into the semiconductor material, causing significant recombination of the charge carriers in this area. In extreme cases fracturing of the semiconductor material may even occur.
Furthermore, in experminents which led to the present invention, the Applicants assembled a plurality of such device strips in parallel on a common substrate to form a two-dimensional sensing area. In order to reduce the insensitive area between the parallel strips, it is desirable for the strips to be closely spaced together. In order to simplify the imaging system using such parallel strips, it is also usually desirable for the readout means and biasing electrode means to be substantially aligned in directions substantially perpendicular to the strips. This double desire for close spacing and alignment can be met by wire-bonding directly to the readout means on each strip, but this suffers from the disadvantages described above.